The long-range goal of this research is to understand the molecular basis of cell proliferation and how it is deranged in cancer cells. The focus of this grant is on the molecular and biochemical aspects of cell cycle control at the G2/M transition in Xenopus oocytes and eggs. Extracts from metaphase II-arrested eggs can be induced to cycle between mitosis and DNA synthesis by addition of free calcium, which mimics the natural signal at fertilization. This cycling reflects periodic changes in cyclin synthesis and degradation,. and periodic activation of the Cdc25 phosphatase by phosphorylation. One specific aim of this grant is to identify the protein kinases that phosphorylate Cdc25 since they may be mitotic triggers. Evidence supports Cdc2/Cdk2 as able to phosphorylate Cdc25, forming a positive feedback loop, but a distinct kinase for Cdc25 phosphorylation has been identified (PIx1) and the kinase that phosphorylates and activates this kinase has also been characterized (xPIkk1). Further work will examine the regulation, localization, and targets of PIx1 and identify kinases that activate xPIkk1.